Interfacial Phenomena on Polymetric Thin Films

Abstract

Model thin films relevant to various biomaterials applications were used to investigate the interfacial phenomena that occur on polymeric surfaces. Quartz Crystal Microbalance with Dissipation (QCM-D) was used to investigate conformational change of soft materials, adsorption and desorption kinetics of macromolecules and interaction between absorbates and model surfaces. Specifically, this thesis deals with enzymatic activity on this films of cellulose of lignin, and salt- induced changes for surface-grafted neutral thermoresponsive polymers.

It was possible to study in situ and in real time the dynamics of enzymatic activity with the QCM-D. After an initial phase, which was controlled by enzyme binding on the substrate, an increase in the resonant frequency of the sensor was apparent. This increase of frequency was explained by the reduction in effective mass due to enzymatic degradation. The rate at which this degradation occurs was found to depend strongly on the enzyme concentration, temperature and the pH of the solution as well as the mixing conditions within the reaction cell.

QCM-D was also employed to estimate the effect of salt on the volume phase transition of thermoresponsive polymer brushes. Changes in mass and viscoelasticity of grafted poly (N-isopropyl acrylamide) (PNIPAM) layers grafted from QCM-D sensors were measured as a function of temperature, upon contact with aqueous solutions of variable salt concentration. The phase transition temperature of PNIPAM brushes, TC,graft, quantified by using the QCM-D was reduced with increases in the concentration of salt. This phenomenon is explained by the tendency of salt ions to affect structure of water molecules (Hofmeister effect).

The investigation on changes in the hydrophlicity of milled wood lignin films were studied by QCM-D and static contact angle mesurements. After laccase incubation, significant decrease in resonant frequency and increase in energy dissipation was observed. It was demonstrated that the film became less hydrophobic due to the oxidation by laccase. Overall, this thesis demonstrates the potential of QCM-D to characterize the detailed kinetics of enzyme activity and to monitor the conformational changes of polymer brushes grafted on the surface of QCM resonator.